The present invention concerns the recovery of aluminum in the form of a specific acid aluminum sulfate hydrate from an impure sulfuric acid solution of aluminum sulfate.
Numerous processes have been described in which an aluminous material, for example a silico-aluminous material, is attacked by a sulfuric acid solution. The hot solution from such a treatment mainly contains aluminum sulfate, but also free sulfuric acid in excess and other metal sulfates such as iron and titanium sulfates and possibly potassium, sodium and/or magnesium sulfates, the nature and amount of which obviously depend on the composition of the treated material and the conditions of the attack.
Various processes have been proposed for the recovery of elemental aluminum from such solutions, in which the aluminum is precipitated as a sulfate, or chloride, which may then be decomposed in order to produce alumina, or sold.
The literature also mentions many specific sulfates that are produced from such solutions, namely anhydrous neutral sulfate, hydrated neutral sulfates containing 4, 6, 9, 12, 14, 16 or 18 H.sub.2 O, basic sulfates and various acid sulfates, for example with 1 or 0.5 H.sub.2 SO.sub.4 and 12 H.sub.2 O. (See, for example, Journal of Metals, July, 1966, pages 811-818, or Linke, 4th Edition, Volume 1, Van Nostrand Company, Inc., 1958, pages 206 ff.).
Some authors doubt the existence of certain of these sulfates for in general they crystallize slowly and their filterability is poor; the cake resulting from their separation, even by filtration, is impregnated with a large amount of impure sulfuric acid-aluminum sulfate mother liquor which makes analysis inaccurate, even after washing. When one goes from a pure solution to an industrial solution which, as has just been said, contains dissolved sulfates other than of aluminum, the presence of a large amount of mother liquor has, in addition to the separation difficulties resulting therefrom, the disadvantage that undesirable amounts of metal sulfates, other than aluminum sulfates, are left in the separated solid mass, some of which involve complex or expensive operations if they are to be subsequently completely separated from aluminum sulfate. In fact, a total separation is very difficult, for in the majority of cases, it requires many successive operations in order to obtain an alumina which is sufficiently pure for metallurigical purposes.
In U.S. Pat. No. 3,484,196 at an intermediate step in the recovery process disclosed obtains an acid aluminum sulfate having essentially the composition Al.sub.2 (SO.sub.4).sub.3.0.5H.sub.2 SO.sub.4.11-12H.sub.2 O, which sulfate is then subjected to treatment by the process described in U.S. Pat. No. 3,397,951 followed by sulfuric acid washing for the recovery of aluminum in the form of the desired neutral sulfate, namely Al.sub.2 (SO.sub.4).sub.3.5.5H.sub.2 O. However, little if any guidance is provided with respect to the parameters for obtaining as the product the acid aluminum sulfate with a morphology that provides a filter cake than can readily be washed for recovery of an essentially pure acid aluminum sulfate.
U.S. Pat. No. 4,039,615 discloses the preparation of hexagonal plate crystals of aluminum sulfate by a specific recrystallization process whereby a slurry containing precipitated aluminum sulfate crystals is heated to dissolve a portion of the crystals, prior to recrystallization of the sulfate to increase the size of hexagonal plate crystals recovered. However, from an industrial standpoint, particularly when one considers the large volumes of impure sulfuric acid aluminum sulfate solutions to be treated, it will be appreciated that the process disclosed is inherently disadvantageous due to the energy input required to repeatedly heat the slurry between recrystallization steps to improve purity of the sulfate, rather than teaching the use of steps that generally prevent a simultaneous precipitation of aluminum sulfate other than the specific acid aluminum sulfate hydrate desired.
The obtaining of a neutral aluminum sulfate of the formula Al.sub.2 (SO.sub.4).sub.3.5.5-6H.sub.2 0 is disclosed in U.S. Pat. Nos. 3,143,392 and 3,141,743 while directed to studies of various products in an Al.sub.2 (SO.sub.4).sub.3.H.sub.2 SO.sub.4 system, wherein the former patent is directed to a crystallization process with classification and recycling of the "fines" and the latter patent in a broad statement states that the process is generally applicable to all known aluminum sulfates.
However, as is recognized in the art by the considerable patents directed thereto, recovery of a specific substantially pure aluminum sulfate hydrate requires adherence to parameters that are not obvious, without undue experimentation, in view of broad statements as to a process being applicable to all known aluminum sulfates.
The present invention concerns means for obtaining by precipitation a specifically defined acid aluminum sulfate hydrate which can be easily washed by virtue of the crystaline morphology of the filter cake so as to provide a source of relatively pure aluminum from what would otherwise be waste products. It has been found that the acid aluminum sulfate hydrate of the formula: EQU Al.sub.2 (SO.sub.4).sub.3.0.5H.sub.2 SO.sub.4.11 to 12H.sub.2 O
achieves the desired result. Its precipitation requires the predetermined selection of a starting mother liquor and manipulative steps which, in particular, generally prevent a simultaneous precipitation of aluminum sulfates other than the specific acid aluminum sulfate hydrate set forth.
The difficulties experienced in avoiding such an undesired simultaneous precipitation of other aluminum sulfates are great since parameters attendant precipitation of the desired sulfate and undesired sulfates are close, and there exist metastable equilibria states and numerous supersaturated states, for the desired and undesired sulfates and the rates of precipitation of each of the numerous other various possible sulfates, especially when starting from supersaturated impure sulfuric acid aluminum sulfate solutions, are different, which contributes to the simultaneous undesirable precipitation of several sulfates, the mixture of which is difficult to filter and wash.